<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="../css/20.css">
</head>
<body>
<div class="one">
    我是一个盒子
</div>
<br>
用户名：<input type="text" class="two">
用户名：<input type="text" class="three">
<br>
<table cellpadding="0"  cellspacing="0">
    <tr>
        <td>123</td>
        <td>123</td>
        <td>123</td>
    </tr>
    <tr>
        <td>123</td>
        <td>123</td>
        <td>123</td>
    </tr>
    <tr>
        <td>123</td>
        <td>123</td>
        <td>123</td>
    </tr>
</table>
<div class="four">
    内边距就是内容和边框之间的距离
</div>
</body>
</html>